arXiv subjects

Colby Haggerty

Publications and source records attributed to Colby Haggerty.

16 recordsLinked to original sources

arXiv2026

Emergence and Detection of Electron-Scale Current Sheets in Turbulence with MMS Observations and fully kinetic 3D simulations

The solar wind is characterized by turbulence, where a cascade produces intermittent current structures called current sheets (CS) that efficiently dissipate energy into the plasma. These have been studied with in situ spacecraft observations, but single-spacecraft techniques such as the partial variance of increments (PVI) are inherently limited since they lack spatial context. A combined analysis of in situ observations and numerical simulations can provide significant insight into the properties of intermittent structures forming in heliospheric turbulence. Understanding the size and distribution of these structures is crucial in tracing the pathways of energy dissipation and particle energization in space plasma. Using 3D fully kinetic simulations of magnetized turbulence, we identify CS via machine learning and find a complex broken-power-law distribution for the CS widths, where the power-law breaks separate ion-scale CS from electron-scale CS. Electron-scale CS dominate, with widths peaking near $2d_e$. Comparing simulations with MMS data, we test PVI as a CS detector and show it can infer CS scale, though oblique crossings inflate inferred sizes. The prevalence of electron-scale sheets suggests they may contribute to plasma heating in aggregate.

physics.plasm-ph↗

arXiv2026

The Role of Magnetic Reconnection in Energizing Protons and Heavier Ions at the Heliospheric Current Sheet

During near-Sun crossings of the heliospheric current sheet (HCS), Parker Solar Probe (PSP) observed populations of high-energy protons and heavier ions indicating possible energization by magnetic reconnection up to 10s -- 100s keV nucleon$^{-1}$. Here we study ion acceleration by magnetic reconnection at the HCS. To estimate ion energization, we solve the Parker transport equation coupled to a large-scale 2D MHD reconnection simulation. We find that multiple ion species develop power-law distributions with both spectral index and high-energy cutoff $E_{\text{max}}$ consistent with in-situ data. By accounting for the injection physics determined by kinetic simulations, we confirm that the charge-to-mass ratio scales as $E_{\text{max}} \propto (Q/M)^{\alpha}$ with $\alpha \sim 0.8-1.1$, approximately consistent with PSP measurements in the broader range $\alpha \sim 0.6-1.7$. In the limit where ions are injected at the same energy per nucleon, $\alpha$ can be as low as $\sim 0.3$. These findings further support the role of magnetic reconnection in producing high-energy heavy ions at the HCS.

astro-ph.SR↗

arXiv2026

Correlation between active regions' spectra at high radio frequencies and solar flare occurrences

High radio frequencies observations with the Italian network of large single-dish radio telescopes resulted in ~450 solar images between 2018 and 2023 in K-band frequency range (18-26 GHz). Solar radio mapping at these frequencies allows the probing of the Active Regions (ARs) chromospheric magnetic field close to the Transition Region, where strong flares and coronal mass ejection events occur. Enhanced magnetic fields up to 1500-2000 G determine anomalous spectra in the ARs brightness compared to pure free-free emission, due to the addition of a steeper gyro-resonance component also associated with circular polarisation up to ~40%. When a significant AR spectral flattening is detected, the probability of a strong flare occurrence within ~30 hours is high (~89% in terms of statistical precision). Despite an approximate weekly cadence of our observations, only ~12% of strong flares are missed/unpredicted within this time interval. Through a correlation analysis, we assess the trade-off on the sensitivity and the robustness of this physics-based flare forecast method.

astro-ph.SR↗

arXiv2025

Statistics of Current and Vorticity Structures in Relativistic Turbulence

Coherent structures created through turbulent cascades play a key role in energy dissipation and particle acceleration. In this work, we investigate both current and vorticity sheets in 3D particle-in-cell simulations of decaying relativistic turbulence in pair plasma by training a self-organizing map to recognize these structures. We subsequently carry out an extensive statistical analysis to reveal their geometric and structural properties. This analysis is systematically applied across a range of magnetizations ($\sigma$) and fluctuating-to-mean magnetic field strengths ($\delta B_0/B_0$) to assess how these parameters influence the resulting structures. We find that the structures' geometric properties form power-law distributions in their probability density functions (PDFs), with the exception of the structure width, which generally exhibits an exponential distribution peaking around 2 electron skin depths. The measurements show weak dependence on $\sigma$ but a strong dependence on $\delta B_0/B_0$. Finally, we investigate the spatial relationship between current sheets and vorticity sheets. We find that most current sheets are directly associated with at least one vorticity sheet neighbor and are often situated between two vorticity sheets. These findings provide a detailed statistical framework for understanding the formation and organization of coherent structures in relativistic magnetized turbulence, allowing for their incorporation into updated theoretical models for structure-based energy dissipation and particle acceleration processes crucial for interpreting high-energy astrophysical observations.

physics.plasm-ph↗

arXiv2025

Efficient Particle Acceleration in 2.5-Dimensional, Hybrid-Kinetic Simulations of Decaying, Supersonic, Plasma Turbulence

Collisionless, turbulent plasmas surround the Earth, from the magnetosphere to the intergalactic medium, and the fluctuations within them affect nearly every field in the space sciences, from space weather forecasts to theories of galaxy formation. Where turbulent motions become supersonic, their interactions can lead to the formation of shocks, which are known to efficiently energize ions to cosmic-ray energies. We present 2.5-dimensional, hybrid-kinetic simulations of decaying, supersonic, non-relativistic turbulence in a collisionless plasma using the code dHybridR. Turbulence within these simulations is highly compressible; after accounting for this compression by taking the omni-directional power-spectrum of the density weighted velocity field, we find turbulent spectra with power-law slopes of $\alpha \approx -\frac{5}{3}$ for low Mach numbers, in the inertial range, and $\alpha \approx -2$ for high Mach numbers. Ions embedded in the highly supersonic simulations are accelerated to non-thermal energies at efficiencies similar to those seen in shocks, despite being in a non-relativistic regime and lacking the large scale structure of a shock. We observe that particles are accelerated into a power-law spectrum, with a slope of $q \approx 2.5$ in (non-relativistic) energy. We compare these results to those obtained from the theory and simulations of diffusive shock acceleration, and discuss the astrophysical implications of this theoretical work.

astro-ph.HE↗

arXiv2024

Maximum energy achievable in supernova remnants: self-consistent simulations

It has been long believed that oblique and quasi-perpendicular configurations in supernova remnants (SNRs) were inefficient at injecting ions into diffusive shock acceleration (DSA), and that the highest energy Galactic cosmic rays (CRs) must come from parallel or quasi-parallel shocks. However, recent 3D kinetic simulations have shown that high-obliquity shocks can successfully energize particles and produce amplified magnetic fields in the upstream. We aim to investigate the maximum energy particles it is possible to produce in oblique and quasi-perpendicular shocks and whether they are capable of triggering the non-resonant hybrid instability (NRHI). We present a novel setup for hybrid simulations of non-relativistic shocks that use a "faux shock" boundary condition instead of a real shock to significantly reduce the computational cost and that can be run for long enough to study the late-time behaviors of these systems. Our results show that it may be possible for oblique and quasi-perpendicular shocks to transition from early periods of shock drift acceleration (SDA) into DSA at later times, giving particles a brief period of rapid acceleration followed by a long-duration, self-sustaining period of slower energy growth. Furthermore, we find evidence that the NRHI is triggered in the upstream at late times. Oblique and quasi-perpendicular shocks may be an important contributor to high energy CRs, potentially even responsible for the knee in the CR energy spectrum.

astro-ph.HE↗

arXiv2023

A Total Solar Eclipse Earth-Based Mission: Multi-wavelength Observations from Land, Sea and Air to Probe the Critical middle Corona

There has been an unfortunate gap in coronal emission line observations from space in the visible and near IR (V+NIR). Their distinct scientific advantage stems from the dominance of radiative excitation in their formation, whereby their emission can be detected out to several solar radii above the limb. V+NIR emission lines can thus yield the only inferences of the physical properties of the coronal plasma, such as species temperatures, densities, elemental abundances, and speeds along and perpendicular to the line of sight in this critical spatial span. These diagnostics have been demonstrated with decades of unsurpassed high-resolution imaging and spectroscopic observations during total solar eclipses. This white paper calls for dedicated funding for a Total Solar Eclipse Earth-Based Mission for ground, airborne and seaborne observations of the corona during totality for the next decade starting in 2024. The proposed Mission capitalizes on the unique diagnostic potential offered by the V+NIR coronal emission lines for the inference of key plasma parameters over a distance range of at least 5 Rs from the solar surface. This critical coronal space is currently missing from existing and to-be launched coronagraphic instrumentation in the proposed time frame. Multi-site observing platforms for each eclipse would further capture the temporal variability of coronal plasmas over a time span of at least 1 hour, with a temporal resolution of a fraction of a minute. Furthermore, this Mission offers unsurpassed opportunities for the exploration of new technologies for future implementation with coronagraphs. This Mission has a unique significant broader impact for outreach opportunities to engage the public and the younger generations in heliospheric science from an awe-inspiring cosmic event.

astro-ph.IM↗

arXiv2022

Modeling the Saturation of the Bell Instability using Hybrid Simulations

The nonresonant streaming instability (Bell instability) plays a pivotal role in the acceleration and confinement of cosmic rays (CRs); yet, the exact mechanism responsible for its saturation and the magnitude of the final amplified magnetic field have not been assessed from first-principles. Using a survey of hybrid simulations (with kinetic ions and fluid electrons), we study the evolution of the Bell instability as a function of the parameters of the CR population. We find that, at saturation, the magnetic pressure in the amplified field is comparable with the initial CR anisotropic pressure, rather than with the CR energy flux as previously argued. These results provide a predictive prescription for the total magnetic field amplification expected in the many astrophysical environments where the Bell instability is important.

astro-ph.HE↗

arXiv2022

A Kinetic Study of the Saturation of the Bell Instability

The nonresonant cosmic ray instability, predicted by Bell (2004), is thought to play an important role in the acceleration and confinement of cosmic rays (CR) close to supernova remnants. Despite its importance, the exact mechanism responsible for the saturation of the instability has not been determined, and there is no first-principle prediction for the amplitude of the saturated magnetic field. Using a survey of self-consistent hybrid simulations (with kinetic ions and fluid electrons), we study the non-linear evolution of the Bell instability as a function of the parameters of the CR population. We find that saturation is achieved when the magnetic pressure in the amplified field is comparable to the initial CR momentum flux.

astro-ph.HE↗

arXiv2022

Cosmic-ray generated bubbles around their sources

Cosmic rays are thought to escape their sources streaming along the local magnetic field lines. We show that this phenomenon generally leads to the excitation of both resonant and non-resonant streaming instabilities. The self-generated magnetic fluctuations induce particle diffusion in extended regions around the source, so that cosmic rays build up a large pressure gradient. By means of two-dimensional (2D) and three-dimensional (3D) hybrid particle-in-cell simulations, we show that such a pressure gradient excavates a cavity around the source and leads to the formation of a cosmic-ray dominated bubble, inside which diffusivity is strongly suppressed. Based on the trends extracted from self-consistent simulations, we estimate that, in the absence of severe damping of the self-generated magnetic fields, the bubble should keep expanding until pressure balance with the surrounding medium is reached, corresponding to a radius of $\sim 10-50$ pc. The implications of the formation of these regions of low diffusivity for sources of Galactic cosmic rays are discussed. Special care is devoted to estimating the self-generated diffusion coefficient and the grammage that cosmic rays might accumulate in the bubbles before moving into the interstellar medium. Based on the results of 3D simulations, general considerations on the morphology of the $\gamma$-ray and synchrotron emission from these extended regions also are outlined.

astro-ph.HE↗

arXiv2021

The Theory of Efficient Particle Acceleration at Shocks

The recent discoveries in the theory of diffusive shock acceleration (DSA) that stem from first-principle kinetic plasma simulations are discussed. When ion acceleration is efficient, the back-reaction of non-thermal particles and self-generated magnetic fields becomes prominent and leads to both enhanced shock compression and particle spectra significantly softer than those predicted by the standard test-particle DSA theory. These results are discussed in the context of the non-thermal phenomenology of astrophysical shocks, with a special focus on the remnant of SN1006.

astro-ph.HE↗

arXiv2021

Non-resonant Streaming Instability driven by Leptons

Using fully-kinetic plasma simulations, we study the non-resonant (Bell) streaming instability driven by energetic leptons. We identify the necessary conditions to drive it and the differences from the standard proton-driven case in both linear and saturated stages. A simple analytic theory is presented to explain simulations. Our findings are crucial for understanding the phenomenology of astrophysical environments where only electrons may be accelerated (e.g., oblique shocks) or where relativistic pairs are produced (e.g., around pulsar wind nebulae).

astro-ph.HE↗

arXiv2021

Relativistic Asymmetric Reconnection

We derive basic scaling equations for relativistic magnetic reconnection in the general case of asymmetric inflow conditions and obtain predictions for the outflow Lorentz factor and the reconnection rate. Kinetic Particle-in-Cell simulations show that the outflow speeds as well as the nonthermal spectral index are constrained by the inflowing plasma with the weaker magnetic energy per particle, in agreement with the scaling predictions. These results are significant for understanding non-thermal emission from reconnection in magnetically-dominated, astrophysical systems, many of which may be asymmetric in nature. The results provide a quantitative approach for including asymmetry on reconnection in the relativistic regime.

physics.plasm-ph↗

arXiv2020

Dynamical Effects of Cosmic Rays on the Medium Surrounding Their Sources

Cosmic rays (CRs) leave their sources mainly along the local magnetic field; in doing so they excite both resonant and nonresonant modes through streaming instabilities. The excitation of these modes leads to enhanced scattering and in turn to a large pressure gradient that causes the formation of bubbles of gas, CRs, and self-generated magnetic fields expanding into the interstellar medium. By means of hybrid Particle-In-Cell simulations, we show that, by exciting the nonresonant instability, CRs excavate a cavity around their source where the diffusivity is strongly suppressed. This finding invalidates the so far largely adopted flux-tube assumption, under which particles move along magnetic lines even in the nonlinear regime. This phenomenon is general and is expected to occur around any sufficiently powerful CR source in the Galaxy. Our results might provide a physical explanation of the numerous claims of suppressed CR diffusion around Galactic sources such as supernova remnants, pulsar wind nebulae, and stellar clusters.

astro-ph.HE↗

arXiv2019

Hybrid Simulations of the Resonant and Non-Resonant Cosmic Ray Streaming Instability

Using hybrid simulations (kinetic ions--fluid electrons), we test the linear theory predictions of the cosmic ray (CR) streaming instability. We consider two types of CR distribution functions: a "hot" distribution where CRs are represented by a drifting power law in momentum and an anisotropic "beam" of monochromatic particles. Additionally, for each CR distribution we scan over different CR densities to transition from triggering the resonant to the non-resonant (Bell) streaming instability. We determine the growth rates of these instabilities in simulations by fitting an exponential curve during the linear stage, and we show that they agree well with the theoretical predictions as a function of wave number agree. We also examine the magnetic helicity as a function of time and wave number, finding a general good agreement with the predictions, as well as some unexpected non-linear features to the instability development.

astro-ph.HE↗

arXiv2019

Modeling the Saturation of the Bell Instability Using Hybrid Simulations

The nonresonant cosmic ray instability, predicted by Bell (2004), is thought to play an important role in the acceleration and confinement of cosmic rays (CRs) close to supernova remnants. Despite its importance, the exact mechanism responsible for the saturation of the instability has not been determined, and there is no first-principle prediction for the amplitude of the saturated magnetic field. Using a survey of self-consistent kinetic hybrid simulations (with kinetic ions and fluid electrons), we study the saturation of the non-resonant streaming instability as a function of the parameters of both the thermal background plasma and the CR population. The strength of the saturated magnetic field has important implications for both CR acceleration in supernova remnants and CR diffusion in the Galaxy.

astro-ph.HE↗